Method for coding moving picture

ABSTRACT

The present invention relates to a moving picture coding method to enhance a coding efficiency for a moving picture coding sequence by inter mode and direct mode in a B picture using a long-term reference picture. In the present invention, a P picture is coded in inter mode according to scene change. The motion vectors of direct mode are calculated and coded according to the kind of the reference picture pointed by a motion vector of a co-located block in the specified picture.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a moving picture coding system,and more particularly, to a moving picture coding method to enhance acoding efficiency using a long-term reference picture.

[0003] 2. Description of the Related Art

[0004] It is desirable to detect a scene change in a sequence so as tocompress and code a moving picture sequence optimally. This is becausemany video applications, such as in news, sports broadcasting, aclose-up conversation like interview, and multi-point videoconferencing, include repeating scene changes. Such a scene change canoccur in an entire picture or in some area of the picture.

[0005] The digital image coding method may be changed whenever a scenechange is detected. For example, since similarity is very low between apicture in which scene change occurs and a picture in a previous scene,a picture with scene change is coded by intra mode in which a picture iscoded using prediction only from decoded samples within the same picturerather than by inter mode in which a picture is coded by motioncompensation from previously-decoded reference pictures.

[0006] In more detail, a picture in which a scene change occurs in theentire picture, is an intra picture that is coded in intra mode on allblocks. Meanwhile, in case of a picture in which a scene change occursat some area, all blocks within the areas in which scene changes occurare coded in intra mode. Since such intra mode generates more bitscompared with inter mode, a sequence in which scene changes occur veryfrequently has a fatal problem in a low bit rate application.

[0007] Generally, when using a B picture in a moving picture codingsystem, the coding order is different from the displaying order.

[0008]FIG. 1 illustrates a display order in which each picture isdisplayed when using two B pictures. As shown in FIG. 1, an intrapicture I is displayed first among pictures to be displayed. Two Bpictures B1 and B2 are displayed subsequently after the intra picture I.A P picture P3 is displayed after the B pictures are displayed. Asdescribed above, the next steps are performed. In other words, thefourth and fifth B pictures B4 and B5 are displayed after the P pictureP3 is displayed. Subsequently, a P picture P6 is displayed.

[0009] However, the coding order of a digital image is not the same asthe display order. In other words, the P picture is coded prior to the Bpicture.

[0010]FIG. 2 illustrates a coding order in which each picture isdisplayed when using two B pictures. As shown in FIG. 2, if an intrapicture I is coded, the P picture P3 is coded prior to the two Bpictures B1 and B2 that are displayed prior to the P picture P3. Afterthat, P6, B4, B5, P9, B7, B8, P12, B10 and B11 are coded subsequently.

[0011] Here, the B pictures have five modes such as intra mode, forwardmode, backward mode, bi-predictive mode and direct mode. Thebi-predictive mode has two reference pictures. Two reference picturesare all located prior to or after the B picture or one of them islocated prior to the B picture and the other is located after the Bpicture.

[0012] Especially, the direct mode utilizes temporal redundancy tomaintain motion continuity between two adjacent pictures. In otherwords, in the direct mode, the forward motion vector and the backwardmotion vector of the direct mode in the B picture are derived from themotion vector of a co-located block in the subsequent picture locatingjust after the B picture. Such a direct mode does not need overhead bitssuch as motion information so that a bit rate can be reduced.

[0013] Here, the forward motion vector MV_(f) and the backward motionvector MV_(b) of the conventional direct mode are obtained by scalingthe motion vector MV using time distance between pictures when theco-located block in a subsequent picture has a motion vector MV. Inother words, the forward motion vector MV_(f) and the backward motionvector MV_(b) are determined using the following Equations 1 and 2.$\begin{matrix}{{{{Equation}\quad 1}:\quad {MVf}} = \frac{{TRb}*{MV}}{TRd}} \\{{{{Equation}\quad 2}:\quad {MVb}} = \frac{( {{TRb} - {TRd}} )*{MV}}{TRd}}\end{matrix}$

[0014] where MV is the motion vector of the co-located block in thesubsequent picture, MV_(f) is the forward motion vector of the directmode for a B picture, MV_(b) is the backward motion vector of the directmode for the B picture, TRd is a time distance between the subsequentpicture and a reference picture pointed by the motion vector of theco-located block in the subsequent picture, and TRb is a time distancebetween a B picture and a reference picture pointed by the motion vectorof the co-located block in the subsequent picture.

[0015] AS a result, the direct mode is a coding mode to obtain twomotion-compensated blocks using two motion vectors MV_(f) and MV_(b) andobtains a prediction block by averaging or interpolative calculation oftwo motion-compensated blocks.

SUMMARY OF THE INVENTION

[0016] The present invention is directed to a moving picture codingmethod that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

[0017] Accordingly, an object of the present invention is to provide amoving picture coding method capable of enhancing coding efficiency bydirect mode using a long-term reference picture for a B picture.

[0018] Another object of the present invention is to provide a movingpicture coding method capable of reducing the amount of bits using intermode for the picture in which a scene change occurs.

[0019] Additional advantages, objects, and features of the inventionwill be set forth in part in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

[0020] To achieve these objects and other advantages and in accordancewith the purpose of the invention, as embodied and broadly describedherein, a method of determining motion vectors of direct mode in a Bpicture includes the step of: when coding each block in the B pictureusing the direct mode, determining differently motion vectors of thedirect mode for the B picture according to a kind of a reference bufferstoring a reference picture pointed by a motion vector of a co-locatedblock in a specified picture.

[0021] It is desired that the specified picture is one of short-termreference pictures used in coding the B picture.

[0022] The kind of the reference picture is determined using a referencepicture index calculated previously at a co-located block in thespecified picture.

[0023] The reference picture index is stored in a system buffer.

[0024] When a motion vector calculated at co-located block in thespecified picture points to a long-term reference picture, a forwardmotion vector of the direct mode for the B picture is a motion vector ofthe co-located block in the specified picture, and a backward motionvector of the direct mode for the B picture is determined to be zero.

[0025] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0026] When a motion vector calculated at co-located block in thespecified picture points to a short-term reference picture, motionvectors of the direct mode for the B picture are determined by scalingthe motion vector of the co-located block in the specified picture bytime distance between pictures.

[0027] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0028] In another aspect of the present invention, a method ofdetermining motion vectors of direct mode in a B picture comprises thestep of: when coding each block in the B picture using the direct mode,determining differently motion vectors of the direct mode for the Bpicture according to a kind of a reference buffer storing a specifiedpicture.

[0029] The reference buffer includes a long-term reference buffer and ashort-term reference buffer.

[0030] It is desired that the specified picture is one of a short-termreference picture and a long-term reference picture.

[0031] When the specified picture is in the long-term reference buffer,a forward motion vector of the direct mode for the B picture is a motionvector of the co-located block in the specified picture, and a backwardmotion vector of the direct mode for the B picture is determined to bezero.

[0032] When the specified picture is in the short-term reference buffer,motion vectors of direct mode for the B picture are determineddifferently according to the kind of the reference buffer storing areference picture pointed by the motion vector of the co-located blockin the specified picture.

[0033] The kind of the reference picture is determined using a referencepicture index calculated previously at the co-located block in thespecified picture.

[0034] The reference picture index is stored in a system buffer.

[0035] When a motion vector calculated at the co-located block in thespecified picture points to a long-term reference picture, a forwardmotion vector of the direct mode for the B picture is a motion vector ofthe co-located block in the specified picture, and a backward motionvector of the direct mode for the B picture is determined to be zero.

[0036] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0037] When a motion vector calculated at co-located block in thespecified picture points to a short-term reference picture, motionvectors of the direct mode for the B picture are determined by scalingthe motion vector of the co-located block in the specified picture bytime distance between pictures.

[0038] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0039] In another aspect of the present invention, a method of coding aP picture of a moving picture in inter mode comprises the steps of: (a)determining whether a scene change occurs in the P picture; and (b) if ascene change occurs in the P picture, coding the P picture withreference to the long-term reference picture.

[0040] It is desired that the P picture in which a scene change occursis one of a scene cut picture and a partial scene change picture.

[0041] If the P picture in which a scene change occurs is a partialscene change picture, the blocks included in an area in which a scenechange occurs are coded using a long-term reference picture.

[0042] A long-term reference buffer storing the long-term referencepicture is a buffer to store a picture coded before a predeterminedtime.

[0043] If the P picture in which a scene change occurs is a partialscene change picture, the blocks included in an area in which a scenechange does not occur are coded using a short-term reference picture.

[0044] A short-term reference buffer storing the short-term referencepicture is a buffer to store a picture coded after a predetermined time.

[0045] In another aspect of the present invention, a method of coding amoving picture sequence in a moving picture coding system comprises thesteps of: (a) determining whether a scene change occurs in a P picture;(b) if there is a P picture in which a scene change occurs, coding the Ppicture in inter mode with reference to a long-term reference picture;(c) when coding each block in a B picture using direct mode according tocoding order, determining a kind of a reference buffer storing aspecified picture; and (d) calculating motion vectors of the direct modefor the B picture according to the kind of the reference buffer andcoding the B picture in the direct mode.

[0046] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0047] When the specified picture is in the long-term reference bufferin the step (d), a forward motion vector of the direct mode for the Bpicture is a motion vector of the co-located block in the specifiedpicture, and a backward motion vector of the direct mode for the Bpicture is determined to be zero.

[0048] When the specified picture is in the short-term reference bufferin the step (d), motion vectors of direct mode for the B picture aredetermined differently according to the kind of the reference bufferstoring a reference picture pointed by the motion vector of theco-located block in the specified picture.

[0049] The kind of the reference picture is determined using a referencepicture index calculated previously at a co-located block in thespecified picture.

[0050] The reference picture index is stored in a system buffer.

[0051] When a motion vector calculated at co-located block in thespecified picture points to a long-term reference picture, a forwardmotion vector of the direct mode for the B picture is a motion vector ofthe co-located block in the specified picture, and a backward motionvector of the direct mode for the B picture is determined to be zero.

[0052] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0053] When a motion vector calculated at co-located block in thespecified picture points to a short-term reference picture, motionvectors of the direct mode for the B picture are determined by scalingthe motion vector calculated at the co-located block in the specifiedpicture by time distance between pictures.

[0054] The motion vector calculated at the co-located block in thespecified picture is stored in a system buffer.

[0055] The P picture in which a scene change occurs is one of a scenecut picture and a partial scene change picture.

[0056] If the P picture in which a scene change occurs is a partialscene change picture, the blocks included in an area in which a scenechange occurs are coded using a long-term reference picture.

[0057] A long-term reference buffer storing the long-term referencepicture is a buffer to store a picture coded before a predeterminedtime.

[0058] If the P picture in which a scene change occurs is a partialscene change picture, the blocks included in an area in which a scenechange does not occur are coded using a short-term reference picture.

[0059] A short-term reference buffer storing the short-term referencepicture is a buffer to store a picture coded after a predetermined time.

[0060] A short-term reference buffer consists of a first input firstoutput (FIFO).

[0061] The specified picture for direct mode coding in the B picture isone of reference pictures used in coding the B picture.

[0062] It is to be understood that both the foregoing generaldescription and the following detailed description of the presentinvention are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0064]FIG. 1 illustrates a display order in which each picture isdisplayed when using two B pictures;

[0065]FIG. 2 illustrates a coding order in which each picture isdisplayed when using two B pictures;

[0066]FIGS. 3A to 3B flowcharts illustrating a method of coding a movingpicture sequence in a moving picture coding system according to thepreferred embodiment of the present invention;

[0067]FIG. 4 illustrates a method of coding a moving picture sequence inwhich a scene change occurs according to the preferred embodiment of thepresent invention; and

[0068]FIG. 5 illustrates a method of coding a B picture in direct modeaccording to the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0069] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0070] To begin with, before an embodiment of the present invention isdescribed, in moving pictures having a scene change, a picture in whicha scene change occurs entirely in the picture is defined as a scene cutpicture and a picture in which a scene change occurs partially in thepicture is defined as a partial scene change picture.

[0071]FIGS. 3A and 3B are flowcharts illustrating a method of coding amoving picture sequence in a moving picture coding system according tothe preferred embodiment of the present invention. Referring to FIGS. 3Aand 3B, pictures are sequentially inputted from a moving picturesequence (S111).

[0072] Kinds of pictures are determined (S114). In other words, it isdetermined whether the inputted picture is a P picture or a B picture.Here, in this embodiment of the present invention, it is assumed that acoding with respect to an intra picture is completed in advance.

[0073] If a picture is the P picture, it is determined whether or not ascene change occurs in the P picture (S117) Here, the scene change isdetermined by comparing the P picture with a picture (P picture or Bpicture) displayed just before the P picture.

[0074] As a result of the determination in the step S117, if the sceneis changed entirely among the P pictures, the P picture is a scene cutpicture. Meanwhile, if the P picture is determined as the scene cutpicture, a coding is carried out with reference to a long-term referencepicture (S120).

[0075] If the P picture is not the scene cut picture, it is determinedwhether or not the P picture is a partial scene change picture (S123).

[0076] If the P picture is the partial scene change picture, blockscontained in an area in which the scene is changed are coded withreference to the long-term reference picture returning to the step S120(S126).

[0077] Blocks contained in an area in which the scene is not changed arecoded with reference to a short-term reference picture (S129, S132).

[0078] Here, the long-term reference picture is a picture stored in along-term reference buffer, and the short-term reference picture is apicture stored in a short-term reference buffer.

[0079] The short-term reference buffer is provided with a first inputfirst output (FIFO) in which a picture first inputted is outputtedfirst, and the pictures coded before relatively short time are stored inthe short-term reference buffer.

[0080] The pictures coded before relatively long time are stored in thelong-term reference buffer. First pictures of respective scene sets,i.e., an intra picture, the scene cut picture, the partial scene changepicture and the like are stored in the long-term reference buffer.

[0081] If there is not the scene cut picture or the partial scene changepicture in the long-term reference buffer, the picture in which thescene change occurs can be stored additionally.

[0082] Accordingly, as shown in FIG. 4, an intra picture I0 that isfirst scene cut picture of a scene set A1, a first scene cut picture P50of a scene set B1 and a first partial scene change picture P120 can bestored in the long-term reference buffer. Here, scene set is a set ofsimilar pictures. For example, suppose a discussion program that anannouncer appears, a panel A appears, the announcer appears again andthe panel A appears again. The scene that the announcer appears first isscene set A, and the scene that the panel A appears subsequently isscene set B. The scene that the announcer appears again is scene set A,and the scene that the panel A appears again is scene set B. Asdescribed above, when a scene change occurs, the P picture is coded byinter mode to code with reference to a short-term reference or along-term reference picture instead intra mode. This reduces the amountof the bits to enhance coding efficiency.

[0083] Description of the step S117 to S132 will be made with FIG. 4. Asshown in FIG. 4, if the P picture P200 to be coded now is the scene cutpicture belonging to the scene set B2, the short-term reference picturesstored in the short-term reference buffer are not used. It is becausethe scene cut picture P200 is the first picture of the scene set B2, andthe scene set of the scene cut picture P200 is different from theshort-term reference pictures such as P199, P198, P197, etc belonging tothe scene set A2. So the similarity of the scene cut picture P200 andthe short-term reference pictures belonging to the scene set A2 isgreatly reduced and the precise coding cannot be achieved from suchreference pictures.

[0084] In this case, the P picture is coded in inter mode with referenceto the other reference pictures P50 and P120 belonging to a scene set B1that is same as a scene set B2.

[0085] On the other hand, if the partial scene change occurs in the Ppicture P250, the coding is performed differently depending on twoconditions. In other words, the blocks included in the area which apartial scene change occurs is coded in inter mode with reference to thelong-term reference pictures P50 and P120 stored in the long-termreference buffer. The blocks included in the area which a partial scenechange does not occur is coded in inter mode with reference to theshort-term reference pictures P249, P248, P247, etc stored in theshort-term reference buffer.

[0086] As described above, after one P picture is coded, the nextpicture is inputted (S159). If the corresponding picture is a B picture,the five prediction modes (intra mode, forward mode, backward mode,bi-predictive mode and direct mode) are tested and one of them isselected as an optimal coding mode (S135, S138). In this specification,direct mode will be described mainly.

[0087] First, one block of the B picture is read (S141). Of course, theother blocks can be read subsequently. After then, a kind of a referencebuffer storing a specified picture is examined

[0088] The specified picture is determined of the earlier pictures thanthe B picture in the coding order regardless of the display order. Inother words, the specified picture is one of the reference pictures usedto code the B picture. Therefore, the specified picture cab be ashort-term reference picture or a long-term reference picture. Theshort-term reference pictures may be before or after the B picture indisplay order and they are stored in the short-term reference buffer.The long-term reference pictures are stored in the long-term referencebuffer. If the specified picture is a long-term reference picture, theforward motion vector of direct mode for the B picture is a motionvector of the co-located block in the specified picture. The backwardmotion vector of direct mode for the B picture is determined to be zero(S150). However, if the specified picture is a short-term referencepicture, the reference picture index and the motion vector calculated atthe co-located block in the specified picture are read (S144). Thesereference picture index and the motion vector is calculated previouslyand stored in the system buffer. According to reference picture index,it is determined whether the motion vector of the co-located block inthe specified picture points to a long-term reference picture (S147). Asdescribed above, the reference pictures are stored in the referencebuffer which includes the short-term reference buffer and the long-termreference buffer.

[0089] If the motion vector of the co-located block in the specifiedpicture points to the long-term reference picture, the B picture iscoded using the following expressions 3 and 4 (S150).

[0090] Expression 3

MVf=MV

[0091] where MV is a motion vector of the co-located block in thespecified picture, and MVf is a forward motion vector of direct mode forthe B picture

[0092] Expression 4

MVb=0

[0093] where MV is a motion vector of the co-located block in thespecified picture, and MVb is a backward motion vector of direct modefor the B picture

[0094] In other words, if the motion vector of the co-located block inthe specified picture points to the long-term reference picture, theforward motion vector of direct mode for the B picture is the motionvector of the co-located block in the specified picture and the backwardmotion vector is zero.

[0095] As shown in FIG. 5, in the step S150, if the motion vector of theco-located block in the specified picture P200 points to the long-termreference picture P50, TRd and TRb is meaningless in the conventionalexpressions 1 and 2. In other words, since TRd and TRb is the timedistance including even the other scene set A2 between the specifiedpicture P200 belonging to the scene set B2 and the long-term referencepicture P50 belonging to the same scene set B1, the forward motionvector and the backward motion vector of direct mode cannot becalculated using such TRd and TRb.

[0096] Referring to FIG. 5, more detailed description is made. Wheninserting two B pictures into a moving picture sequence and coding them,the P picture P200 that is earlier than the B1 and B2 pictures in codingorder is coded at first. Here since the P picture P200 is a scene cutpicture in which a scene change occurs, the P picture P200 is coded ininter mode from the long-term reference picture P50 stored in thelong-term reference buffer. According to the coding order, the nextpicture to be coded is a B1 picture. Since the B1 picture belongs to ascene set A2, the most blocks are coded in forward mode from theshort-term reference pictures belonging to the scene set A2 or inbi-predictive mode in which all of two reference pictures belong to thescene set A2. However, intra mode, backward mode or bi-predictive modefrom the P picture P200 belonging to the other scene set B2, and directmode to obtain motion vectors of the direct mode from the co-locatedblock in the P picture P200 are probably not used as the coding mode forthe blocks in the B1 picture.

[0097] Differently, since not only the B2 picture but also the specifiedpicture P200 used formotion vectors of direct mode for the B2 picturebelong to the same scene set B2, the direct mode is selected as a codingmode for the most blocks in the B2 picture. In other words, afterobtaining motion vector of each block in the specified picture P200 byinter mode from the long-term reference picture P50 belonging to thesame scene set B2, the motion vectors of direct mode in the B2 pictureare calculated from the motion vector of the co-located block in thespecified picture P200. Since the B2 picture and the specified pictureP200 belong to the scene set B2, the long-term reference picture P50also belongs to the scene set B1, and the similarity between the sceneset B1 and the scene set B2 is very high, the direct mode can beselected as a coding mode for the most blocks in the B2 picture.Accordingly, the coding efficiency for the B2 picture is improved.

[0098] On the other hand, if the motion vector of the co-located blockin the specified picture points to a short-term reference picture, the Bpicture is coded using the conventional Expressions 1 and 2. In thistime, since the short-term reference picture stored in the short-termreference buffer belongs to the same scene set as the B picture belongsto and other scene set does not exist between the specified picture andthe short-term reference picture, the forward motion vector and thebackward motion vector of direct mode are determined using theconventional Expressions 1 and 2 related to TRd and TRb representingtime distance.

[0099] If one block of B picture is coded, the next block in the Bpicture is read and coded subsequently (S156). Such processes areperformed on all blocks in the B picture. After the B picture is coded,the next picture is inputted and coded so that a moving picture codingis achieved (S159).

[0100] As described above, according to a moving picture coding methodof the present invention, the forward motion vector and the backwardmotion vector of direct mode for the B picture are determineddifferently based on the reference picture pointed by the motion vectorof the co-located block in the specified picture. When coding the Bpicture, direct mode is mainly used as coding mode to enhance the entirecoding efficiency.

[0101] According to the moving picture coding method of the presentinvention, the P picture in which a scene change occurs is coded ininter mode using motion compensation from a long-term reference toreduce the amount of bits and enhance coding efficiency.

[0102] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of determining motion vectors of directmode in a B picture, the method comprising the step of: when coding eachblock in the B picture using the direct mode, determining differentlymotion vectors of the direct mode for the B picture according to a kindof a reference buffer storing a reference picture pointed by a motionvector of a co-located block in a specified picture.
 2. The methodaccording to claim 1, wherein the specified picture is one of short-termreference pictures used in coding the B picture.
 3. The method accordingto claim 1, wherein the kind of the reference picture is determinedusing a reference picture index calculated previously at the co-locatedblock in the specified picture.
 4. The method according to claim 3,wherein the reference picture index is stored in a system buffer.
 5. Themethod according to claim 1, wherein when a motion vector calculated atthe co-located block in the specified picture points to a long-termreference picture, a forward motion vector of the direct mode for the Bpicture is a motion vector of the co-located block in the specifiedpicture, and a backward motion vector of the direct mode for the Bpicture is determined to be zero.
 6. The method according to claim 5,wherein the motion vector of the co-located block in the specifiedpicture is stored in a system buffer.
 7. The method according to claim1, wherein when a motion vector calculated at the co-located block inthe specified picture points to a short-term reference picture, motionvectors of the direct mode for the B picture are determined by scalingthe motion vector of the co-located block in the specified picture bytime distance between pictures.
 8. The method according to claim 7,wherein the motion vector of the co-located block in the specifiedpicture is stored in a system buffer.
 9. A method of determining motionvectors of direct mode in a B picture, the method comprising the stepof: when coding each block in the B picture using the direct mode,determining differently motion vectors of the direct mode for the Bpicture according to a kind of a reference buffer storing a specifiedpicture.
 10. The method according to claim 9, wherein the referencebuffer includes a long-term reference buffer and a short-term referencebuffer.
 11. The method according to claim 9, wherein the specifiedpicture is one of a short-term reference picture and a long-termreference picture
 12. The method according to claim 9, wherein when thespecified picture is in the long-term reference buffer, a forward motionvector of the direct mode for the B picture is a motion vector of theco-located block in the specified picture, and a backward motion vectorof the direct mode for the B picture is determined to be zero.
 13. Themethod according to claim 9, wherein when the specified picture is inthe short-term reference buffer, motion vectors of direct mode for the Bpicture are determined differently according to the kind of thereference buffer storing a reference picture pointed by the motionvector of the co-located block in the specified picture.
 14. The methodaccording to claim 13, wherein the kind of the reference picture isdetermined using a reference picture index calculated previously at theco-located block in the specified picture.
 15. The method according toclaim 14, wherein the reference picture index is stored in a systembuffer.
 16. The method according to claim 13, wherein when a motionvector calculated at the co-located block in the specified picturepoints to a long-term reference picture, a forward motion vector of thedirect mode for the B picture is a motion vector of the co-located blockin the specified picture, and a backward motion vector of the directmode for the B picture is determined to be zero.
 17. The methodaccording to claim 16, wherein the motion vector of the co-located blockin the specified picture is stored in a system buffer.
 18. The methodaccording to claim 13, wherein when a motion vector calculated atco-located block in the specified picture points to a short-termreference picture, motion vectors of the direct mode for the B pictureare determined by scaling the motion vector of the co-located block inthe specified picture by time distance between pictures.
 19. The methodaccording to claim 18, wherein the motion vector of the co-located blockin the specified picture is stored in a system buffer.
 20. A method ofcoding a P picture of a moving picture in inter mode, the methodcomprising the steps of: (a) determining whether a scene change occursin the P picture; and (b) if a scene change occurs in the P picture,coding the P picture with reference to the long-term reference picture.21. The method according to claim 20, wherein the P picture in which ascene change occurs is one of a scene cut picture and a partial scenechange picture.
 22. The method according to claim 20, wherein if the Ppicture in which a scene change occurs is a partial scene changepicture, the blocks included in an area in which a scene change occursare coded using a long-term reference picture.
 23. The method accordingto claim 22, wherein a long-term reference buffer storing the long-termreference picture is a buffer to store a picture coded before apredetermined time.
 24. The method according to claim 20, wherein if theP picture in which a scene change occurs is a partial scene changepicture, the blocks included in an area in which a scene change does notoccur are coded using a short-term reference picture.
 25. The methodaccording to claim 24, wherein a short-term reference buffer storing theshort-term reference picture is a buffer to store a picture coded aftera predetermined time.
 26. A method of coding a moving picture sequencein a moving picture coding system, the method comprising the steps of:(a) determining whether a scene change occurs in a P picture; (b) ifthere is a P picture in which a scene change occurs, coding the Ppicture in inter mode with reference to a long-term reference picture;(c) when coding each block in a B picture using direct mode according tocoding order, determining a kind of a reference buffer storing aspecified picture; and (d) calculating motion vectors of the direct modefor the B picture according to the kind of the reference buffer andcoding the B picture in the direct mode.
 27. The method according toclaim 26, wherein the motion vector of the co-located block in thespecified picture is stored in a system buffer.
 28. The method accordingto claim 26, wherein when the specified picture is in the long-termreference buffer in the step (d), a forward motion vector of the directmode for the B picture is a motion vector of the co-located block in thespecified picture, and a backward motion vector of the direct mode forthe B picture is determined to be zero.
 29. The method according toclaim 26, wherein when the specified picture is in the short-termreference buffer in the step (d), motion vectors of direct mode for theB picture are determined differently according to the kind of thereference buffer storing a reference picture pointed by the motionvector of the co-located block in the specified picture.
 30. The methodaccording to claim 29, wherein the kind of the reference picture isdetermined using a reference picture index calculated previously at aco-located block in the specified picture.
 31. The method according toclaim 30, wherein the reference picture index is stored in a systembuffer.
 32. The method according to claim 29, wherein when a motionvector calculated at co-located block in the specified picture points toa long-term reference picture, a forward motion vector of the directmode for the B picture is a motion vector of the co-located block in thespecified picture, and a backward motion vector of the direct mode forthe B picture is determined to be zero.
 33. The method according toclaim 32, wherein the motion vector of the co-located block in thespecified picture is stored in a system buffer.
 34. The method accordingto claim 29, wherein when a motion vector calculated at co-located blockin the specified picture points to a short-term reference picture,motion vectors of the direct mode for the B picture are determined byscaling the motion vector calculated at the co-located block in thespecified picture by time distance between pictures.
 35. The methodaccording to claim 34, wherein the motion vector of the co-located blockin the specified picture is stored in a system buffer.
 36. The methodaccording to claim 26, wherein the P picture in which a scene changeoccurs is one of a scene cut picture and a partial scene change picture.37. The method according to claim 26, wherein if the P picture in whicha scene change occurs is a partial scene change picture, the blocksincluded in an area in which a scene change occurs are coded using along-term reference picture.
 38. The method according to claim 37,wherein a long-term reference buffer storing the long-term referencepicture is a buffer to store a picture coded before a predeterminedtime.
 39. The method according to claim 26, wherein if the P picture inwhich a scene change occurs is a partial scene change picture, theblocks included in an area in which a scene change does not occur arecoded using a short-term reference picture.
 40. The method according toclaim 39, wherein a short-term reference buffer storing the short-termreference picture is a buffer to store a picture coded after apredetermined time.
 41. The method according to claim 39, wherein ashort-term reference buffer consists of a first input first output(FIFO).
 42. The method according to claim 26, wherein the specifiedpicture for direct mode coding in the B picture is one of referencepictures used in coding the B picture.